The present invention relates to a developing method for visualizing an electrostatic latent image formed on an image-bearing member surface with a magnetic developer including magnetic toner in an electrophotographic method, an electrostatic recording method, an electrostatic printing method, etc.
In a typical electrophotographic method, an electrostatic latent image formed on a photosensitive drum surface is visualized with colored resin particles called toners, and the resulting toner image is fixed to a transfer sheet such as a plain paper by heating or pressurizing means to obtain a fixed image.
Various methods of developing such an electrostatic latent image have been proposed so far. Widely used among them is a method in which magnetic toner consisting of toner particles comprising a resin and magnetic powder is supplied onto a non-magnetic sleeve, to form a magnetic brush by a relative rotation of the sleeve and a permanent magnet member disposed inside the sleeve, and an image-bearing member surface is in slidable contact with this magnetic brush, thereby permitting the toner particles to attach to the electrostatic latent image. In this one-component magnetic toner system, a chargeable magnetic toner, which contains a charge-controlling agent so that the toner may be strongly charged in a particular (positive or negative) polarity, is used, and the development of an electrostatic latent image is carried out by utilizing a triboelectric charging phenomenon due to the contact of the toner particles with a sleeve or a doctor blade member.
However, when the chargeable magnetic toner is used alone, the toner particles are likely to be agglomerated by electric charging, so that streaks are formed on the image due to a shortage of toner on the sleeve. To obviate this problem, it has been proposed to use a developer comprising magnetic toner and magnetic carrier (U.S. Pat. Nos. 4,517,274 and 4,640,880).
In such a method of developing an electrostatic latent image by using a developer comprising a chargeable magnetic toner and a magnetic carrier, a magnetic carrier having a relatively high conductivity is used, and the toner concentration is not particularly controlled. Accordingly, the toner concentration is in a wide range of 10-90% in a developing region. Also, since the magnetic toner has an insulating property from the viewpoint of transferability, the electric resistance of the entire developer is extremely high. Thus, in order to obtain an image having a high image density and a good quality by using such a developer, efficient development of an electrostatic latent image is required. For this purpose, at least one of the non-magnetic sleeve and the magnet roll is usually rotated at a high speed to increase a speed of conveying the developer, thereby improving the development efficiency. However, when either one of the above members is rotated at a high speed, the developer is likely to be severely damaged, and large noises tend to be generated in the driving system.